Radiothyroxine kinetics in yearling brook trout, Salvelinus fontinalis (Mitchill), on different levels of dietary intake

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Abstract
Groups of yearling brook trout maintained on a 12 h light: 12 h dark photoperiod regime and held between 10.5 and 12.5.degree. C were either fed once daily 2.5 or 5.0% of their body wt (dry weight of food (grams)/dry weight of fish (grams)) or were starved for 37 days. All fish were then injected into the circulation via the heart with 1.5 .mu.Ci (1 Ci = 37 GBq) of 125I-labeled L-thyroxine (*T4; specific activity 37 .mu.Ci/.mu.g). Fed fish received their prescribed rations 18 h before and 4, 29, 53 and 77 h after *T4 injection. Starvation lowered the T4 metabolic clearance rate (MCR), plasma T4 level, T4 degradation rate (T4 DR), extent of *T4 deiodination, and conversion of *T4 to *T3 (125I-labeled 3,5,3''-triiodo-L-thyronine). Relative to trout on the 2.5% ration, those on the 5.0% ration had a higher fractional turnover of *T4 in plasma, a smaller T4 distribution space, lower MCR, higher plasma T4 levels, greater T4 DR, and more extensive conversion of *T4 to *T3. Different caloric and (or) nutrient intakes have a pronounced effect on T4 kinetics and conversion of T4 to T3 in brook trout.
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